Alternating current (AC) electric motors are a type of electric motor used in a variety of applications such as in electric vehicles. Electric motors are also commonly used in products to drive devices such as fans, pumps, and compressors, and in manufacturing processes to drive conveyor belts, saws, and other machinery. Some AC electric motors can also operate as a generator, such as an automobile alternator.
Electric motors include a stator component encircling a rotor component. For the power generation function, electric current is selectively supplied to wire windings of the stator, causing alternating electromagnetic fields within the stator.
The rotor has highly-conductive rods, such as a high-conductive aluminum, and the electromagnetic fields created in the windings of the stator induce current in the bars of the rotor, and interaction between the magnetic fields and the current in the bars cause the rotor to rotate, thereby generating power in the turning rotor. The turning rotor can provide drive to an electric or hybrid vehicle, a fan, pump, compressor, and other equipment.
Induction motors are sometimes referred to as asynchronous motors because an effective speed of a magnetic field created by the stator must be different than a speed of the rotor to induce current in the conductor bars of the rotor, and thereby cause the rotor to rotate.
To guide current through the stator as desired, the wound wires forming windings are surrounded by layers of insulation, which prevent electrical short circuits between the wires, turn-to-turn, phase-to-phase and turn-to-phase, and in some cases between the wires and other electrical features of the motor or other equipment adjacent the motor. The insulation can include any of a variety of materials and configurations. The insulation typically comprises an organic or non-organic dielectric material. The wires can be connected to the insulation in a variety of ways, such as by dipping the wires in a bath of the insulation or introducing the wires to slot liners, and then the wires and liners are introduced to a lamination stack. The lamination stack includes individual thin laminations of insulation material.
Wires are typically isolated from the lamination stack by the slot liners to avoid damaging the wires during insertion. If the lamination stack has sharp edges and burrs, it could lead to partial discharge, or Corona, when the wires inside are carrying current. Partial discharges are sometimes colloquially known by the acronym, PD.
If insulation is not comprehensive around the wires, electrical discharges can result from interaction with adjacent wires of the winding or between the exposed wire and other features of the motor or environment in which the stator is used. This interaction usually involves direct contact between the uncovered wires and another wire or conductor.
The insulation can be damaged as a result of a variety of manufacturing errors. For example, insulation materials could fail during insertion of the wires through slots of a lamination stack. The insulation material on the wires could also fail during bending of the wires to form the winds of the stator. In some cases, insufficient insulation is applied to a wire during a dipping process. In some cases, damage caused during the manufacturing process (such as pin hole, inclusions during the coating process, poor adhesion, etc.) does not produce an immediate failure of the insulation, but weakens the insulation to an extent that a premature failure occurs during the service life of the motor. When the insulation is defective, an electric field in the wire increases significantly at an outer surface of the wire.
If the defect is large enough, and so the concentrated electrical field at the defect is large, electricity will discharge from the wire in an electrical discharge. More particularly, a corona or partial discharge are photons being emitted in response to ionization of air or a local high-electric field. In cases of significant damage to insulation, the electrical discharge will be large enough to form of an electrical arc, causing the motor to fail by short circuit. In cases where insulation has been damaged to a lesser degree, lower-magnitude, or partial, discharges can result, generating a corona that typically does not itself cause the stator to fail. Repeated coronas, though, can progressively damage the insulation, eventually leading to failure of the motor.
It is therefore a goal of the presently disclosed technology to anticipate coronas at an early stage in a manufacturing process by detecting a problem with wire insulation, a slot liner, etc.
Coronas generate audio noise and radio interference that is not visible in the light because it is mostly in the ultraviolet (UV) range. On new motors, a corona generally indicates that one of various common problems are present, such as a faulty assembly, a problem with wire insulation, a problem with liner insulation, a problem with stator lamination slot quality, or a problem with wire bending. Hence, the present technology includes apparatus and methods for checking for corona soon after a motor component is assembled, locating any identified corona therein, and taking advised actions early, such as before the motor component is incorporated into a motor assembly, and before an arc or short are formed.
Photon detection systems are available commercially for motor inspection. These systems typically include a camera being manually employed to determine whether a partial discharge has occurred. Ultraviolet camera systems have also been used to detect corona events. These systems, though, are limited to identifying only corona discharges that are visible to the camera. That is, these systems cannot recognize discharges that occur deeper within the stator because the resulting ultraviolet radiation does not reach a surface of the stator. These systems, though, are not able to identify a particular location of the partial discharge in the component.